Press molding of a metal plate material is conventional working method used in manufacturing of automobiles, machines, electric equipment, transport equipment, etc. due to its high productivity and high-precision working ability. Recently, an increase in the strength of steel plate, for example, as a material for automobile parts has been advanced in terms of reduction in the weight of parts. In press molding of a high-tensile steel plate, a problem that springback, wrinkling, etc. may occur, which can cause defective shapes would likely manifest. Furthermore, an increase in the strength of the metal plate material causes increase in the pressure of a contact surface with a mold at the time of press molding, which can raise a problem that a frictional force between the mold and the metal plate material may exceed the withstand load of a lubricant oil to thereby cause a defective surface due to die galling or the like and damage the mold. In this manner, the productivity may consequently be reduces.
In order to prevent the occurrence of molding defects such as crack, wrinkling, and galling of the metal plate material after press molding, a method may be used for forming plural recesses in part or all of the surface of the mold and confining the lubricant oil between the surface of the mold and the metal plate material to thereby improve a sliding property, as described in Japanese Patent Application Laid-open No. Hei 6-210370. However, this method may have a problem in that if the friction force increases because of the increase in the strength of the metal plate material, a sufficient lubricating effect may not be obtained.
Moreover, when a metal plate material with low press moldability is molded, a hot press molding method of heating the metal plate material and pressing it at a high temperature can be effective. In this hot press molding, the cooling of the metal plate material after molding in terms of productivity may be of importance. Accordingly, a method for cooling with a refrigerant after press molding at a high temperature can be used, as described in Japanese Patent Application Laid-open No. Hei 7-47431 and Japanese Patent Application Laid-open No. 2002-282951.
However, the method described in Japanese Patent Application Laid-open No. Hei 7-47431 is used to supply air from an air output provided at a peripheral portion of a punch of a warm press mold, and perform cooling with the air with low heat capacity and heat conductivity as a medium> Such method may have difficulty in changing the air with air existing in a gap between the mold and the metal plate material, and thus can possess a problem of a low cooling efficiency. Furthermore, the method described in Japanese Patent Application Laid-open No. 2002-282951 is generally used to define a clearance between the mold and the metal plate material, provide refrigerant introducing grooves in a molding surface of the mold which touches the metal plate material; and increase the cooling rate using the refrigerant. However, when the refrigerant flows into the refrigerant introducing grooves, the temperature at the outlet side can become higher than that at the inlet side, and the refrigerant becomes difficult to flow along the grooves due to deformation of the metal plate material at the time of molding, which makes uniform cooling difficult. Additionally, there may be a problem that a continuous groove shape tends to be transferred to the molded metal plate material.
Accordingly, there is a need to overcome at least some of the above-described deficiencies.